1. Field of the Invention
The present invention relates to a method for detecting liquid leaks from a variety of systems such as chemical plants, tanks, pipelines for reserving and conveying liquids, and also relates to a cable therefor.
2. Description of the Prior Art
Many kinds of liquid chemicals are stored in tanks and transported between plants through pipelines. This has widely been done in industries. The liquid transportation through pipeline has been preferentially used because of its higher efficiency and lower expense compared with that of the conventional method of liquid transport, tank lorry and tank wagon. However, a number of leaks and leak-related accidents have taken place in the storage and/or transportation systems. The numbers of petroleum pipeline accidents in the United States during the period 1968-1970 are, for example: outer surface corrosion (517, 42%), mishap (258, 21%), inferior pipe seam (92, 7.4%), inner surface corrosion (63, 5.0%), mis-operation (40, 3.2%), gasket breakage (35, 2.8%), valve trouble (25, 2.0%), old scar (25, 2.0%), over surge pressure (18, 1.5%), mis-welding (17, 1.4%), others (bad sealing, flood, land slide, mis-coupling, etc.; 144, 11.7%); 411 accidents per year on the average.
In addition, the liquid chemicals leaked constitutes not only danger of explosion, but also environmental pollution, so that the leak detection is very important from both the standpoint of security and environmental preservation. In Japan, in constructing petroleum pipelines, it is necessary by law, ministerial ordinance and notification to install security and watch systems, such as automatic operation control, emergency alarm, pump stopper, etc. However, a leak detecting method having enough sensitivity and reliability has not yet been established.
Explanation will be made here on the conventional leak detecting systems:
1. Pressure Measuring Method during Operation
This method is commonly used in countries outside of Japan, and corresponds to the method defined in Japanese Ministerial Ordinance, Article 29, and Notification Article 41. The method comprises placing pressure gauges at an appropriate interval along a pipeline, measuring pressures at each gauge, comparing the measured pressures with the pressure gradient previously calculated and thus detecting pressure drop due to liquid leak. This is suitable for detecting a sudden and large amount of liquid leak, but unsuitable for detecting a small leak, and has problems in pressure alteration due to the change of liquid type and pipeline temperature.
2. Pressure Difference Method on Stop of
Operation
In this method, all block valves in a pipeline are closed at one time, and the pressure difference between both sides of each valve is measured. When a leak occurs in a given section between two valves, pressure differences are produced in both valves. In this method, however, a pressure difference is also generated by a slight temperature difference between adjacent sections (e.g., a temperature drop of 0.1.degree.C results in a pressure drop of 1 kg/cm.sup.2), so that it is necessary for this system to pre-anticipate pressure alteration due to geological and meterological variations. Thus the accuracy of this method is impaired. Furthermore, the slow rate of leak detection of this method (e.g.; 3 days for detecting 10 1/hr leak) occupies about 10% of total time of pipeline operation (90% working ratio). This method is specified in the Japanese Ministerial Ordinance, Article 32 and Notification, Article 43, Paragraph 3, and in West Germany this method is compelled by law to be employed in the beverage industries.
3. Static Pressure Difference Method
This is similar in principle to the above method 2 except that a pressure drop is measured between two valves. This method can detect a large leak (more than 500 l/hr) within 15 minutes total test time, but cannot detect a small leak.
4. Flow Amount Measuring Method
This method detects a leak by measuring flow amounts by flowmeters placed at inlet and outlet ports, and then comparing the amounts. For this purpose, two kinds of flowmeters, a flash type for flow control and an integrating flowmeter for measuring a transport amount, are needed. The Ordinance, Article 27, and Notification, Article 41 state that alarm shall be made when the flow amount difference exceeds 80 l per 30 sec. This method, however, has the following defects: (a) Since the pipe expansion and contraction due to pressure change with the flow starting and stopping cannot be neglected, the flow measurements could include errors, if the pressure changes have previously been estimated: (b) When transporting different types of oil through a pipeline, the liquid flow is rapidly altered with the valve opening/shutting and pump operation on changing the liquid: (c) Turbine type flowmeters are generally employed in this method, but they produce errors owing to wear with time, hence require periodical inspection.
5. Ultrasonic Method
This method detects a pipeline leak by sensing ultrasonic waves generated by the pressure difference at a leak point. This sensing is performed by an apparatus which flows inside the tube and is equipped with an ultrasonic receiver, amplifier, amplitude recorder, range meter, and battery. There are two types of ultrasonic detectors based on this principle; i.e., Royal Dutch Shell Type and Esso Research and Engineering Co., Type. The Esso Type detector is actually used in the crude oil pipeline (pipe dia. 40 inches) from Trieste, Italy to Ingor Stadt, Germany. This method has a limit in leak detecting ability (i.e., the maximum sensitivity being 0.1 to 0.4% of the total flow), is unsuitable for a pipeline with a small diameter, and is expensive.
6. Gas Sensor Method
The arrangement of this method, stated in Japanese patent publication Sho 48-11134, comprises an oil resistant fibrous layer having oil and air permeability wound around a pipeline, a small diameter tube having pores contained in the fibrous layer and connected to a valve box located in the pipeline, and a gas sensor placed in the valve box. A leak is detected by sensing a leaked gas led to the sensor through the small tube. A compressor connected to the tube enables faster leak detection. This method also requires increased expenses in installation and construction, and the sensitivity thereof is not so sharp.
In addition to the prior processes mentioned above, the utilization of a cable has already been proposed for liquid leak detection. The cable is a pair of insulated conductors, which insulation material is dissolved by a leaked liquid, so that the two conductors are shortcircuited to indicate the leak. An example thereof is a polystyrene-insulated cable for detecting gasoline leak. This type of cable, however, has a danger for ignition of leaked liquid due to a spark generated by shortcircuiting, so that it is quite unsuitable for a pipeline conveying flammable liquids.